When would you use the Observer design pattern?

When an object's state change should notify its dependents
When complex object creation is abstracted and centralized
When objects should be able to change behavior at runtime
When there is a one-to-many dependency between objects

The Observer design pattern is used when there is a need for a one-to-many dependency between objects, where one object's state change should notify and update multiple other objects. This pattern is commonly used in event handling systems and UI frameworks.

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